fix(telemetry): no default value for telemetry.enable in schema

the default value is computed from version string

apps/emqx_telemetry/src/emqx_telemetry_schema.erl

@@ -30,7 +30,7 @@
 roots() -> ["telemetry"].
 
 fields("telemetry") ->
-    [{enable, ?HOCON(boolean(), #{default => true, desc => ?DESC("enable")})}].
+    [{enable, ?HOCON(boolean(), #{required => false, desc => ?DESC("enable")})}].
 
 desc("telemetry") ->
     ?DESC("telemetry_root_doc");